The present invention relates to torque sensors for measuring the relative rotational movement between two connected shafts. More specifically, the present invention relates to a torque sensor adapted for inclusion in an electric power assisted steering system.
Many modern automobiles include a power steering system. A variety of systems are known, including those operating on principles of hydraulics and those that utilize electric motors to assist steering. Regardless of the operational principle, the effect is the same: steering of the vehicle by a driver is easier due to a lower torque threshold needed to effect steering.
In the past, hydraulic power steering systems have been the predominant type of system incorporated in motor vehicles. Unfortunately, these systems have proved inefficient, primarily due to the need to size components needed to respond to steering demands under a broad range of operating conditions.
Electric power assisted steering (EPAS) systems present a more efficient system, and have become an area of rapid development. In these systems, an electric motor provides force to a shaft or a steering rack of the vehicle to assist in turning the wheels of the vehicle. Just as with a hydraulic system, the EPAS system reduces the input torque needed to effect steering of the vehicle.
One problem facing increased acceptance of EPAS systems is the need to package system components within the space typically occupied by conventional hydraulic system components. Accordingly, there is a need for a torque sensing device with improved manufacturability.
The present invention provides a torque sensing device with components optimized to meet packaging constraints typically present in motor vehicles.
In one embodiment, the torque sensing device comprises first and second shafts. The first shaft defines a projection and the second shaft defines a recess. The recess is adapted to receive the projection and to allow a limited degree of rotational movement of the projection. A torsion bar interconnects the first and second shafts and allows rotational movement of the shafts relative to each other. A bushing is disposed around the torsion bar, and is frictionally engaged with the projection. A plurality of magnets is circumferentially disposed about the bushing. A stator assembly is fixedly mounted to the second shaft and includes first and second poles. One or more sensing devices are disposed in the clearance between the first and second poles. Protective plates can be disposed within the clearance and on first and second sides of the sensing device.